Problem: $ { {5} \times \left[\begin{array}{rr}{1} & {-1} \\ {3} & {1}\end{array}\right] = {?} } $
To multiply a matrix by a number, multiply each item of the matrix by the number. ${\left[\begin{array}{rr}{5}\times{1} & {5}\times{-1} \\ {5}\times{3} & {5}\times{1}\end{array}\right]}$ Now, simplify: ${\left[\begin{array}{rr}{5} & {-5} \\ {15} & {5}\end{array}\right]}$